Process for the creation of a thermal SiO2 layer with extremely uniform layer thickness

ABSTRACT

Disclosed is a reproducible process for making an SiO 2  layer by thermal oxidation which assures an extremely uniform thickness of the SiO 2  layer of approximately 1%. The process of the invention comprises the steps growing an initial layer of SiO 2  to a defined minimal thickness by dry oxidation and increasing the thickness of the initial layer by simultaneous wet and dry oxidation until the desired final thickness is reached.

The invention relates to a process for making an SiO₂ layer by thermaloxidation and in particular to a reproducible process which assures anextremely uniform thickness of the SiO₂ layer.

Thermally oxidizing silicon surfaces is one of the key process steps formanufacturing micromechanical as well as semiconductor devices. In thesemiconductor manufacturing process sequence the critical oxidation stepis producing the gate oxide layer for field effect transistors. Theseextremely thin oxide layers, a typical thickness range is 10 nm to 20nm, have to fulfill high quality requirements and therefore the thermaloxidation for these layers is done with dry oxygen which provides theknown small oxidation rate. Using dry oxidation thickness uniformitiesof these thin layers which are in the range of about 1% to 4% may beachieved.

For thin oxide layers the slow dry oxidation may be tolerable, but notfor oxide layers in the range from 0.1 μm up to several μm which areusual for the silicon-on-insulator technology and for various types ofmicromechanical devices like e.g. calibration standards as disclosed inthe application EP 94 10 5568 filed Apr. 11, 1994.

Especially for the calibration standard the uniformity of the siliconoxide layer should be as best as possible since it directly affects theaccurateness of the standard.

Variations in either the temperature or wetness of the various waferswhich are being processed, caused by variations in the cleaningprocedure and/or the air to which the wafers are exposed, causevariations in the rate of initial heating and oxidation of the wafersand thereby cause significant large variations in the amount of oxidegrowth, especially in the spontaneous oxide layer. To eliminate theseeffects in U.S. Pat. No. 3,892,891 the wafers are pre-heated at amoderate temperature, e.g. in the range between 100° and 200° C., in acontrolled atmosphere, e.g. air having a controlled dew point.

Todays' furnace manufacturer guarantee uniformities of 1% for dry oxygenand 2.5% for wet oxygen processes. Assuming a desired layer thickness of1 μm this would mean uniformity differences of about 10 nm to 30 nmwhich is not sufficient for e.g. micromechanical calibration standards.

It is an object of the present invention to provide a reproducibleprocess for the creation of a thermal SiO₂ layer which shows an extremeuniformity in the layer thickness of approximately 1%.

FIG. 1 illustrates an example for the thickness variation of a siliconoxide layer.

FIG. 2 illustrates a temperature/time profile of a preferred processsequence.

The process of the invention comprises the steps growing an initiallayer of SiO₂ to a defined minimal thickness by dry oxidation andincreasing the thickness of the initial layer by simultaneous wet anddry oxidation until the desired final thickness is reached.

This process offers the possibility to reproducibly manufacturerelatively thick oxide layers with uniformities better than oxide layersof the same thickness made with dry oxygen only. The uniformity of oxidelayers produced according to the claimed process and having a thicknessof 0.5 μm was ±0.37 nm along a 50 mm diameter line in the center of a100 mm wafer. An example for the thickness variation of a 0.5 μm siliconoxide layer is given in FIG. 1.

Further preferable solutions and embodiments are apparent from thedependent claims.

One way of carrying out the invention is described in detail below withreference to FIG. 2 which shows the temperature/time profile of aspecial process sequence.

In FIG. 2 there is shown the temperature/time profile of a preferredprocess sequence of the invention with the various phases 0 to 15.

Phase 0 represents the loading of the oxidation chamber, which typicallyis a quartz tube, with semiconductor wafers or other substrate materialwhich is to be coated with silicon dioxide. This loading step takesplace at uncritical low temperatures of about 600° C. and is followed byfilling the oxidation chamber with inert media, preferably nitrogen, atthis temperature during about 15 minutes with a flow rate of about 10l/min. Step 1 helps stabilizing the temperature in the oxidation chamberand assures the chamber being completely filled with nitrogen whichprevents the oxidation process starting at this temperature.

In steps 2 and 3 the temperature in the oxidation chamber is increaseduntil a defined first low oxidation temperature is reached. The firstquick ramping step with a temperature increase of about 7° C./min and aflow rate of 2 l/min nitrogen stops shortly before reaching the firstlow oxidation temperature and with the second slow ramping step withabout 1° C./min the desired first low oxidation temperature finally isreached. Typically the first low oxidation temperature is in the rangebetween approximately 600° C. and approximately 850° C.

A nitrogen flow of 2 l/min at this first low oxidation temperature instep 4, during about 10 min, stabilizes the temperature in the oxidationchamber for the next important step 5 in which the dry oxidation of aninitial layer starts. The nitrogen is quickly removed from the oxidationchamber by a defined flow of dry oxygen of about 10 l/min. This highflow rate assures the quick exchange of nitrogen and oxygen in theoxidation chamber at the first low oxidation temperature. Step 5 assuresa controlled and slow start of the dry oxidation step for an initiallayer at the desired first oxidation temperature.

After the quick exchange of inert and oxidizing media in the oxidationchamber in step 6 oxygen with a defined low flow rate enters theoxydation chamber. Preferably the flow rate is as low as 0.5 l/min. Theexchange of nitrogen and oxygen being completed, during step 6 thetemperature in the oxidation chamber is again slowly increased from thefirst low oxidation temperature to a defined second oxidationtemperature with about 1° C./min. The second oxidation temperature ishigher than the first low oxidation temperature and is in the rangebetween approximately 950° C. and approximately 1050° C.

These two steps, step 5 and step 6, at the beginning of the dryoxidation process, have great impact on the thickness uniformity of thesilicon dioxide layer to be formed. They assure the controlled,homogeneous and slow start of the dry oxidation step at the relativelylow first oxidation temperature. Of similar importance is the quickexchange of nitrogen and oxygen and the low flow rate of oxygen afterthe exchange being completed. Together with the low first oxidationtemperature this keeps low the growth rate of the thermal dry oxidelayer and provides a high quality oxide layer.

In step 7 the second oxidation temperature is kept constant for about 4hours during which dry oxidation occurs until a defined minimalthickness of the initial layer is reached. The defined minimal thicknessof the initial layer of SiO₂ is independent from the desired finalthickness. It is in the range between approximately 100 nm andapproximately 200 nm.

During the following steps the thickness of the initial layer will beincreased by simultaneous wet and dry oxidation until the desired finalthickness is reached.

In step 8 the temperature in the oxidation chamber is slowly increasedby about 1° C./min with about 0.5 l/min oxygen until a defined thirdoxidation temperature is reached. The third oxidation temperature ishigher than the first oxidation temperature and is in the range betweenapproximately 950° C. and approximately 1050° C.

For reasons of temperature stabilization the third oxidation temperatureis kept constant during about 10 min in step 9 with 0.5 l/min oxygenflow before the simultaneous wet and dry oxidation starts in step 10 atthe third oxidation temperature.

By separate inlet means a defined flow of water vapour and a definedflow of oxygen, preferably 0.5 l/min, are simultaneously provided intothe oxidation chamber during a predetermined second time period,preferably of about 1 hour 25 min. During this time period the thirdoxidation temperature is kept constant.

Another alternative possibility to reach the desired final thickness isto start with step 10 directly after step 7, that is running thesimultaneous wet and dry oxidation at the second oxidation temperatureduring the predetermined second time period.

In step 11 the simultaneous wet and dry oxidation process is stopped byproviding only oxygen into the oxidation chamber and keeping constantthe second or third oxidation temperature for further approximately 10min.

Thereafter the controlled downramping in several steps begins. Thesesteps also have some impact on the thickness uniformity since during atleast some of the downramping steps further oxide growth is to beexpected.

With about 1° C./min in step 12 the second or third oxidationtemperature slowly decreases until approximately the first low oxidationtemperature is reached and during this step of decreasing the oxidationtemperature a defined flow of dry oxygen of about 0.5 l/min is providedinto the oxidation chamber.

In step 13 the oxidation process is stopped in a controlled manner. Thelow oxidation temperature is kept constant during about 10 min to allowthe quick exchange of oxygen and nitrogen with a nitrogen flow rate ofapproximately 10 l/min.

The second downramping step 14 further decreases the temperature with 3°C./min and a nitrogen flow rate of about 2 l/min until a temperaturesuitable for deloading the oxidation chamber is reached.

In the example described above the desired final thickness of thesilicon dioxide layer produced was 500 nm. This thickness was reachedduring the dry, wet/dry and dry oxidation steps 5 to 13. The thicknessuniformity of the 0.5 μm silicon dioxide layer produced according to theprocess described above is shown in FIG. 1. Along a 50 mm diameter linein the center of a 100 mm wafer there were only thickness variations assmall as ±0.37 nm to be found.

What is claims is:
 1. A process for the creation of a thermal SiO₂ layerwith extremely uniform layer thickness, the process comprising the stepsof:growing an initial layer of SiO₂ to an initial thickness by dryoxidation; and increasing the thickness of said initial layer bysubsequent oxidation until the desired final thickness is reached,wherein said step of growing an initial layer of SiO₂ further comprisesthe steps of:providing a inert media into an oxidation chamber;increasing the temperature in said oxidation chamber until a first lowoxidation temperature is reached; starting said dry oxidation at saidlow oxidation temperature by removing said inert media from saidoxidation chamber and providing dry oxygen into the oxidation chamber;increasing the temperature in said oxidation chamber from a first lowoxidation temperature to a second oxidation temperature, said secondoxidation temperature being higher than said first low oxidationtemperature; and keeping constant said second oxidation temperature insaid oxidation chamber for a first time period, and wherein saidincreasing the thickness of said initial layer further comprises thesteps of:increasing the temperature in said oxidation chamber until athird oxidation temperature is reached, said third oxidation temperaturebeing higher than said first oxidation temperature; starting saidsubsequent oxidation at said third oxidation temperature; and keepingconstant said third oxidation temperature in said oxidation chamber fora second time period.
 2. The process of claim 1 wherein after increasingthe thickness of said initial layer the process further comprises thesteps of decreasing said third oxidation temperature until said firstlow oxidation temperature is reached and during said step of decreasingsaid oxidation temperature providing dry oxygen into the oxidationchamber.
 3. The process of claim 1 wherein said first low oxidationtemperature is in the range of approximately 600° C. to approximately850° C., said second oxidation temperature is in the range ofapproximately 950° C. to approximately 1050° C. and said third oxidationtemperature is in the range of approximately 950° C. to approximately1050° C.